Telephone and like transmission system



May 21, 1935. A. D. BL UMLEIN TELEPHONE AND LIKE TRANSMISSION SYSTEM Filed April 24, 1950 4 Sheets-Sheet 1 W ls 0.000000000000000.

' FIG-3.

May 21, 1935. A. D. BLUMLEIN TELEPHONE AND LIKE TRANSMISSION SYSTEM Filed April 24, 1930 4 Sheets-Sheet 2.

Zo= ZAZB FIG 6.

y 1935. A. D. BLUMLEIN 2,001,900

TELEPHONE AND LIKE TRANSMISSION SYSTEM Filed Apfi] 24, 1930 4 Sheets-Sheet s 921 06 &. 29 e A nuuunuu 1 May 21, 1935. A. D. BLUMLEIN I S TELEPHONE AND LIKE TRANSMISSION SYSTEM Filed April 24, 1930 4 Sheets-Sheet 4 .IOIOOIIIIU\II uuunuuu Patented May 21, 1935 TELEPHONE AND L LIKE TRANSMISSION SYSTEM A Alan Dower Blumlein, London, England, assignor to Columbia Phonograph Company,

Inc.,

Bridgeport, Conn., a corporation of New York Application Apr-i124, 1930. serial No. 446,892

In Great Britain June 29, 1929 i 1 Claim. (01. 118-44) This invention relates to electrical transmis-,

sion systems with particular reference to amplifiers, sound reproducing systems and telephone transmission lines and the like over which complex frequencies are transmitted.

It is known that in order to correct for, or introduce distortion in magnitude or phase in an electrical transmission system use may be made of networks which are inserted in the system in order to modify the relations between the input currents and output currents at various frequencies. The object of these networks may be (1) to improve the quality of the transmission system by modifying the relative magnitudes and phases of certain frequencies; (2) to cut out or. reduce certain frequencies which may not be desired as they introduce noise or overloading; (3) to control the magnitude of the received currents to a desired value; '(4) to produce standards of attenuation against which apparatus may be compared; (5) to select certain frequencies for transmission for testing purposes. These networks may also be used to effect economies by correcting for distortion of magnitude and phase produced by systems of transmissionwhich, though cheap, cannot be used Without correction.

The main object of the present invention is to combine efficiency of transmission with economy in the cost of installing the system by reducing thecompiexity of networks employed while at the same time permitting such networks properly to fulfill their functions. Y 1

According to my invention I employ what may be termed a Half Wheatstone network,.which 35: may be arranged to perform any of the functions stone bridge accordingto the invention.

Fig. 2 shows a further form. a Fig. 3 represents a telephone phantoming repeating coil group. q =Fig.4 represents a modified arrangement of .Fig.3.. .p 1 Fig. shows a device similar to Fig. 2..- 1:

Fig. 6 represents an arrangement having a uniform transmission and equivalent to Fig. 5.

Fig. '7 illustrates a network according to the invention and designed to have constant: characteristic impedances and an attenuation variable with frequency. V

Fig. 8 illustrates a network having an imperfect transformer.

Fig. 9 shows a combination of four networks according to the invention, while i i 1 Fig. 10 shows-a combination of five networks.

Figs. 11 to 14. show further forms of the inven-i tion in which a transformer or retardation coil is employed whose-tapping point is not central; and Fig. 15 shows a still further form of the invention. H

The equations giving the current in the detecting device of a Wheatstone bridge in terms 'of the E. M. F. of, orcurrent'supplied by, the source 'are well known. It isalso well known that by varying the impedance of the arms of the bridge the current flowing to the detecting'device can be varied.

Accordingto the form' of the invention shown in Fig. 1 Iemploy two ratio arms I and. 2 consisting of two similar windings on a commonhigh permeability magnetic core, while the other twoarms of the bridge consist of two impedances ZA and ZB. The detector terminals 3 .are located across the ratio arms and impedances while the generator'terminals 4 are connected to'the junc tions of the two ratio arms and of the impedances respectively. A modification of the bridge may be employed as shown in'Fig.*2 in which the detector terminals 3 are not connected directly to the bridge but are connected to a. secondary transformer winding 5 on the-same magnetic core as the windings I and 2. The generator and detector are of course interchangeable without affecting the bridge balance. If the electrical characteristics of windings l and 2 are equal then no current will beobtainedyat the detector terminals 3 located across the ends of the secondary transformer winding from analternating source at the generator terminals 4 arranged as before, if the impedance of Z. is equal to that of 23.

If Zn does not equal Z3 the power will be transmitted through the bridge from the source to the detector. By varying the degree of unbalance Of'ZA and Z3 the phase and magnitude of-the transmitted currents can be varied at will. Finally by so constructing ZA and Zn that their degree of unbalance varies with frequency the bridge may be used to transmit currents with. a

' definite frequency discrimination.

This bridge network may be used as a network for modifying currents transmitted through it from the generator terminals 4 to thedetector terminals 3 or vice versa. V

the phantom circuit, that is to say no power can be transmitted between these terminals, but if however ZA and ZB are unbalanced then power can flow between such terminals. .If, now. the terminals 8 of the phantom circuit are short circuited and a generator applied to terminals Sand.

ZA and ZB are unbalanced, then current will flow in the phantom circuit. If, then Z'A and 'Z'B are unbalanced this phantom current will cause currents to flow in the second side circuit in an impedance connected to terminals 1. Thus the currents transmitted from terminals 6 and I y can be modified twice, first by the unbalance of ZA and ZB and then by the unbalance of 2'1; and

' Fig. 4 shows the circuit above described rearranged with the terminals-.8 of the phantom circuit shortcircuited, whereby power may be transmitted from terminals 6 as side circuit tel:- minals through the unbalance of gZA and Z3 to the phantom circuit at and back to another side circuit through the unbalance of ZA and 2']; to terminals 7. It will be seen that if the network is divided along the dotted line II it is the same as two of the networks shown in Fig. 2.

Having thus described two view points from which the method of operation of the network may be understood some descriptionof the impedance and attenuation characteristics will now be given. a t V .7,

Fig. 5 illustrates a .Half Wheatstone network according to they invention comprising a primary winding l6 of a transformer and a secondary winding having two. balanced halves Hand l8. This transformer is, for the purpose of the following calculations, assumed to be perfect, within the working frequency-range of the device. That is to say, assuming that L1, L2 and M be the inductances of, and mutual between the windings, then M is assumed very large and un-ca ,1

is assumed very small; and the resistance of the windings is assumed to be negligible.

Also, the centre point tap 20 of the secondary is assumed to be such that for-a current entering at the point 29 and di'viding'and flowing out equally at ends of the secondary winding 2| and 22 n0 impedance will be presented to this current and no potential difference will be produced between terminals l2 and I3 of the primary winding or' between the. ends of the secondary winding 2| and 22. Z may be assumed to be the impedance looking into terminals l2 and I3 of the primary winding and Z the impedance looking into the terminals 14 and 15 connected. respectively to the: center of: the secondary windingand toimpedances ZA and ZB arranged in. series one with each half of the secondary winding. The impedance Z when the terminals 14 and I5 are short circuitedmay be written Zsc, and when on open" circuit Zw By applying Kirchhoffs laws to this network it can be easily shown that Z'OC=ZA+ZB and The characteristic impedance looking into the terminals 12 and 13 of the primary winding may then be written Where 0 is the transfer or total propagation constant of the network.

Similarly the impedances lookingv in at the terminals of the secondary windings M and [5 using the same notation are given by:-

and as before II sc A 1 ZB then V The network is therefore equivalent to the arrangement shown in Fig. 6 having a uniform transmission line 24 with impedance Z.=1/zAZB and total propagation constant given by and having two ideal transformers 23 and 25 having voltage ratios of /Ezl and impedance ratios of 2:1.

The characteristic impedances between terminals l2 and I 3 of the first transformer and between the terminals l4 and I5 of the second transformer will then as before be given by Z 'o ZZd and Bysuitably proportioning ZA and ZB the network may" be given a desired impedanceand a desired attenuation and. phase characteristic; Since Z13. it can be shown that so the values of 2A and zBmay be calculated for any desired values of Zn and 9. 'This network may be made to perform many hence functions, a few examp O which are j i below. 7 i For example ZA and Z3 may be pure resistances in which case 9 will be real and constant with Further by arranging the switches (or a slide wire) to make V I Z4 ZB both greater and less than unity an attenuator may be produced which will give an output current in either sense, i. e. reverse the current direction.

Figure 7 shows a network designed to have. constant characteristic impedances and an attenuation variable with frequency. One of the arms of the bridge is provided with a resistance R. and inductance L in series, while the other arm is provided with a resistance R shunted by'a capacity C. The values of L and C are chosen desired fashion.

will be equal to It will therefore be seen that as the frequency is'changed from 0 to w the attenuation decreases from .to 0 and the phase constant changes from V to 1r; 7

The arrangement is therefore a network of constant characteristic impedance and attenuation varying with'frequency. By making ZA and ZB of other arrangements of resistances inductances and condensers, other types of variation of attenuation and phase constant with frequency can be obtained with and without constant impedances.

It is also possible to produce variable attenuation networks having the impedance varying in a suppose zs. consists of an inductance L and Zn of a capacity C the impedance will be constant,

This'gives value of propagation constant given Theratio f :,12' mm/Ti Then this network has constant impedance, no

' attenuation and a phase constant changing from 0 to 211' as frequency increases from 0 to Anetwork of this type may therefore be used for correcting phase relations in a system. More complicated types may of course be made using combinations of inductances, condensers, and. resistances for the two arms.

If ZA is composed of an inductance L and capacity C in series, and 213 of an inductance L, a network is produced having no. attenuation for frequencies below a certain limit. In this case That is to say that when w the network has no attenuation but only a phaseconstant but when w w1the network gives attenuation without phase shift,in fact the network is acting as a low pass filter. The impedence of this network is not constant, being given by For frequencies below cut off frequency w t1 this impedance is resistive and for low frequencies approximates to V JET C Above out 01f the impedance is reactive. As

21 having an ideal transformer, in circuit with an imperfect transformerZE similar to that actually employed. in the network though no use is made of the center tapping 29. This transformer is connected to the high impedance side of the network. 28 represents animpedance Zp which is equal to the impedance presented by thetransformer secondary to theflow of current from the center tapping 29 of the secondary winding dividing equally and leaving by terminals 3% and S] of the winding. Zp is in-fact the impedance of the two halves of the secondary winding connected in parallel opposing, This arrangement is then identically equivalent to a half Wheatstone network shown in Fig. 5 having an imperfect transformer. The network 2'! representing I a network having anideal transformer may as- I As has been stated before a resistance 50 is inbefore be replaced by the network as shown in Fig. 6 without altering the characteristics.

If an imperfect retardation coil is employed instead of a transformer in the network, this network may be represented by an arrangement similar to that shown in Fig. 8 except that the transformer 26 is replaced by the imperfect retardation coil and the impedance Zp is the impedance of the two retardation coil windings in parallel opposing.

In order to obtain a good balance on the transformer or retardation coil and at the same time reduce the value. of Zp to a minimum it is advantageous to wind the two balanced halves as tightly coupled as possible; for example, intermeshed or bifilar windings may be used similar to those usually employed for the line windings of telephone repeating coils where an accurate central phantom tapping is required. In order to balance the capacities of the coil it may be advantageous to use an electrostatically shielded transformer or to earth the center point tapping.

If tightly coupledwindings are used for the two balanced halves of the coil the impedance Zp will be practically a, pure resistance. In this case if the impedance of the network is resistive, a shunt resistance and further series resistance may be added in order that Zp may become one arm of a T network of impedance giving a constant attenuation invariant with freenormoquency. The impedance at terminals. I 4 and [5,

Figure 8,. will then be exactly I Similarly if a similar type of network is to be i inserted at I4 and I5 so that in effect two re- Such resistance is shown at 40, Fig. 9. Although this resistance increases the attenuation it makes the impedance, conditions correct at this junction even though imperfect transformers are used.

If it is desired to obtain equal impedances at each endof a single section of this network a 2 :1 impedance ratio transformer may be used. Similarly any ratio may be used to obtain a desired impedance at the end of the network.

A number of these networks may be connected together in order that their attenuative effects may add together. Fig. 9 shows four sections of network may be employed connected in succession and having three transformers. By making Z0 the same for each of the networks the attenuations of the networks will add directly. The impedance at the terminals 36 and 3'! of the primary of one transformer, between sections 33 and 34, and at the terminals 38 and 39 of the last transformer will be 220. The impedance between sections 32' and 33 and between sections 34 and 35 will be serted tocorrect the error in impedance relations introduced "between sections 35 and 35 by the parallel opposing impedances of the two halves of the tapped transformer windings. The

effect of this is to introducea constant loss independent of frequency.

A more complex method of coupling is employed where five'sections of network are utilized, as illustrated in Fig; 10. The network comprises five sections M, 42, 43, 44, and 45 having terminals 46 and 41. In order to make the impedance relations correct The characteristic impedance at the terminals 46 of the first section will then be ideal as possible.

. 2,001,900- tions 42, 43 and 44 must be low, in order that.

the direct coupling produced between thev input of and the output of 44 by the parallel opposing impedance of the retardation coils shall be negligible in eifect.

Section is coupled to Section 45 by means of a retardation coil. Due to the method'fof coupling it is necessary that An earth connection is provided in section 43 at 48 and it will be seen that the coupling between sections 43 and 44 is such that both input and output leads of section 44 have one side atground potential. Such coupling might be used between the anode of a three electrode vacuum tube and the grid of a successive tube.

Many other arrangements of these networks in succession can be devised, and these networks may also be joined in serieswith networks of other well-known types. V

In order that this network may be as effective as possible it is necessary that the transformer or retardation coil usedshould be as nearly It is especially important that the loss introduced by the transformer should be constant throughout the working range of the device, or this variation of attenuation will be added to the attenuation variation with frequency of the network.

Any improvements in transformer design which improve the impedance relations seen through the transformers and decrease the variation of loss with frequency will improve the device, so also will any improvements which reduce the impedance of the two balanced windings in parallel opposing. Improvements in the balance of the transformer or retardation coil'will improve the operation of the device at high attenuations.

Another possibility arises in. the use of a transformer or retardation coil whose tapping point is not central. This tapping point may be moved slightly from center in order to allow for small manufacturing errors in the constituents of the two impedance arms.

This type of network may also be made having unequal ratio arms, that is to s'ay'when the tap-' ping point of the transformer is. not central. The two windings forming the secondary or retardaticn coil are then wound with the turns in a definite ratio, not being unity. The ratio, assuming the two windings to be perfectly tight coupled, would be proportional to the number of turns. Figs. 11 and 12 show such a network having a positive value of 11 which is the ratio of the coil winding turns. Fig. 11 represents the arrangement employing a retardation coil, and Fig. 12 and arrangement employing a transformer. Furthermore, the ratio n may have negative value as shown in Figs. 13 and 14 and may be given by either a retardation coil, Fig. 13 or a transformer, Fig. 14. In such cases the ratio windings approximate to perfectly tight coupled windings having n times the number of turns between the terminals 49 and a of one coil as there arebetween the terminals 58 and 5| of the other coil.

The windings are connected so as to give an inductive series aiding connection when connected as shown. When a transformer is used, as shown in Figs. 12 and 14, it is assumed, for the formulae which follow, that the primary winding 5 has the same number of turns as there are between the two points 49 and 5|. The armimpedances in the network are taken as ZA and n ZB so that the bridge is balanced when ZA=ZB.

For thesefnetworks the. characteristic impedance Zu looking in at the terminals 3 of the primary is given by r and the characteristic impedance Z0 looking into the terminals 4 from the tapped secondary and the impedances. is given by It will be seenthat when ZA=ZB, tanh- 0:1, that is to say the attenuation'is infinite.

The. expression for 0 may be re-written by putting n Z13.=Zc-whereZc istheimpedance in thearm."

For example suppose ZA is an inductance L and Z0 is a capacity C such that 0 is then given by:-

w A d- Q12 1 V 1 x 7 It will be seen that when there will be no attenuation since tanh 0 will be imaginary and so 0 imaginary. Also when there will be attenuation since tanh' 0 will be real and so 0 real.

If n is negative there will be a frequency of infinite attenuation when A further method of using the network with equal ratio arms consists of interchanging the network impedance and the input and output terminals. Such an arrangement is shown in Fig. 15'. The two network impedances are shown as 2ZA bridged across the transformer secondary 52 and As for the network shown in Fig. 5

tails are given purely by way of illustration and not of limitation since the nature of the invention only is herein indicated and I may vary the number of networksemployed, the manner in which the respective impedances are obtained and the values of the inductances, resistances and capacities adopted depending upon the purpose for which the network is to be employed or any practical requirements that may have to be fulfilled.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

An electrical transmission net-work, comprising two inductive windings tightly coupled to each other and two impedances, said inductances and impedances being adjacent each other and being connected to form a bridge, a pair of terminals connected to the junction point of said impedances and to the junction point of said inductances, another pair of terminals connected to the extreme ends of said induotances and a resistance shunt connected across said first-named pair of terminals to correct the impedance change introduced by the parallel inductive opposing impedance of the magnetically coupled arms.

ALAN DOWER BLUMLEIN. 

